1. Field of the Invention
This invention relates to a drive control method of an image pickup element that can be applied to an image pickup system for picking up an image of an object moving at high speed. Such a system typically find applications in the field of factory automation (FA).
2. Prior Art
It is a known practice that, when a signal receiving device receives a signal transmitted from a transmitter in order to drive a circuit in the receiver to operate according to the received signal, the receiver is required to prepare a signal that meets the requirements of the circuit by referring to the received signal. For such a purpose, the receiver is typically provided with a pulse generation circuit and, as annex, a pulse width selection circuit having a switch. With such an arrangement, so that the user of the receiver can select a pulse width that matches the specifications of the receiver on the basis of the signal transmitted from the transmitter for the purpose of controlling the operation of the circuit to be driven of the receiver by means of the desired pulse.
Examples of such systems that are designed to control the operation of the signal receiving side on the basis of an externally transmitted signal include video camera systems. In video camera systems, necessary video information can be taken out at high speed by transferring unnecessary signals at high speed so as to sweep them out, while transferring necessary signals by way of a normal transfer process and processing them to obtain effective pixels. This function is often referred to as high rate function. For operating a video camera by using the high rate function, it is necessary to input a reference signal that indicates the start of a high speed transfer period and also a control signal that indicates the ongoing transfer period is required.
The inventor of the present invention proposed in Japanese Patent Application Laid-Open Publication No. 10-191177 a drive control method of an image pickup element, an image pickup element, an image pickup control device and an image pickup system with which an image pickup operation is conducted by controlling the effective electric charge accumulation time of an interline transfer (IT) type solid state image sensor (CCD image sensor), thereby realizing high speed random shuttering in synchronism with a trigger signal so that an effective electric charge of a predetermined image pickup range can be obtained as image pickup signal.
Referring to FIG. 1 of the accompanying drawing illustrating an image pickup system according to the above identified invention, an electric charge read signal that is generated at a predetermined timing on the basis of a trigger signal EXT-TRG is used to read out the electric charges of the picked up image accumulated in a plurality of light receiving elements of the interline transfer type solid state image sensor to a vertical transfer section. Then, the electric charges of the picked up image read out to said vertical transfer section are vertically transferred at a high transfer rate in synchronism with vertical synchronizing pulse VD and subsequently the electric charges of the picked up image are read out as image pickup signal from said vertical transfer section by way of a horizontal transfer section at an ordinary transfer rare in synchronism with a vertical transfer signal HD. With the image pickup system, the effective electric charge of a voluntarily selected range of image can be obtained as image pickup signal by selecting a high speed transfer period and changing the number of lines output as image pickup signal.
Meanwhile, in known image pickup systems, an externally input control signal as illustrated in FIG. 2 is used as reference signal indicating the start of a high speed transfer period and the transfer period is defined by making the pulse width of the control signal variable and using a switch mounted in the camera main body to specify the transfer period. Therefore, if the switch is not of the steplessly variable type, it is not possible to select a transfer period that is less than the minimum control time unit of the switch.
On the other hand, when an image pickup system is remotely operated in a factory, the reference signal that indicates the start of a high speed transfer period is designed to be externally input to the conventional video camera system and hence can be remotely input but the switch mounted on the camera main body has to be operated to define the duration of the transfer period and hence it is not possible to remotely control the switch.
Additionally, for picking up an image of an object moving at high speed without blur, the accumulation period of the CCD image sensor has to be made short. The problem of blur is normally dissolved by using an electronic shutter for the system. Then, the object needs to be illuminated intensely by light or the aperture of the image pickup optical system has to be controlled to offset the reduction in the electric charge due to a short accumulation period in order to secure an appropriate signal output level so that an image may be picked up with the signal level of an ordinary picture as shown in FIG. 3A.
However, with this arrangement, the smear that may be produced to the vertical transfer section that is located adjacent to the light receiving section of the CCD image sensor is not controlled so that, if the image pickup system can select a high speed transfer period or an ordinary transfer period, there arises a time lag between the high speed transfer period and the ordinary transfer period. Then, a phenomenon of overlapped images arises as shown in FIG. 3B by the real image signal produced by the electric charges of the picked up image and the ghost signal produced by the smear of the vertical transfer section. The problem lies in the smear of the vertical transfer section, which changes with the time difference and can affect the signal level of the object to be shot that is found in the effective image pickup span.